[IRGen] swift_getFunctionTypeMetadataGlobalActor has concurrency availability

Fixes rdar://79674106.

Author: DougGregor

include/swift/Runtime/RuntimeFunctions.def

@@ -600,7 +600,7 @@ FUNCTION(GetFunctionMetadataDifferentiable,
 //                                          const Metadata *globalActor);
 FUNCTION(GetFunctionMetadataGlobalActor,
          swift_getFunctionTypeMetadataGlobalActor,
-         C_CC, AlwaysAvailable,
+         C_CC, ConcurrencyAvailability,
          RETURNS(TypeMetadataPtrTy),
          ARGS(SizeTy,
               SizeTy,